1. Field of the Invention
The present invention relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to a design for a defibrillator cell, such as a prismatic cell stack, containing double screen sandwich cathodes. Double screen sandwich cathode electrodes are based on a novel cathode configuration termed a sandwich cathode electrode. The structure of a sandwich cathode electrode will be described in detail hereinafter as well as how it differs from a double screen sandwich cathode electrode of the present invention.
2. Prior Art
Implantable ventricular cardiac defibrillators typically use lithium/silver vanadium oxide (Li/SVO) electrochemical cells as their power source. For the implantable medical device itself, it is preferable that the device be relatively small in size, quick in response to the patient""s medical needs, promote long device service life, etc. Therefore, when cells are built for implantable medical applications, special electrode assembly designs are needed to meet all of these requirements. Additionally, for cells powering cardiac defibrillators, a large electrode surface area is required to provide the needed power capability. An efficient cell package is also needed to achieve the highest capacity in the smallest volume.
In a conventional electrode assembly for Li/SVO cells, the cathode active material is pressed, coated or otherwise contacted to both sides of a foil or screen cathode current collector to provide the cathode electrode. Lithium as the anode active material in the form of a foil is pressed onto both sides of an anode current collector to form the anode electrode. The anode and the cathode electrodes are then placed against each other with one or two layers of intermediate separator material. The final electrode assembly is typically in the form of a prismatic plate design or a jellyroll design. An example of the conventional prismatic plate design is disclosed in U.S. Pat. No. 5,147,737 to Post et al. An example of a conventional jellyroll design is disclosed in U.S. Pat. No. 5,439,760 to Howard et al.
To further illustrate this point, FIG. 1 shows a detailed cross-sectional view of the electrode assembly 10 of either a conventional prismatic plate design or a conventional jellyroll design. The electrode assembly 10 comprises an anode electrode 12 and a cathode electrode 14 physically segregated from each other by separator sheets 16. The anode electrode 12 comprises an anode active material 18, such as lithium, contacted to at least one side of an anode current collector 20. Similarly, the cathode electrode 14 comprises a cathode active material 22, such as SVO or CFx, contacted to at least one side of a cathode current collector 24. Whether the cell is of a prismatic plate or jellyroll configuration, they are typically built in a case-negative configuration with the anode current collector 20 having an outermost position in contact with the casing (not shown) as the anode or negative terminal. The cathode electrode is contacted to a terminal lead (not shown) insulated from the casing by a glass-to-metal seal, as is well known by those skilled in the art.
Depending on the number of plates in the prismatic configuration, or the number of winds in a jellyroll cell, the conventional electrode assembly 10 can have n repeating units of the anode electrode 12 and the cathode electrode 14. This is shown in FIG. 1 where n=0, 1, 2, 3, 4, 5, etc.
U.S. patent application Ser. No. 09/560,060, filed Apr. 27, 2000, which is assigned to the assignee of the present invention and incorporated herein by reference, describes a sandwich cathode electrode design for defibrillator applications. The sandwich cathode electrode design is believed to be a pioneering improvement over the conventional prismatic and jellyroll electrode assemblies. In the sandwich cathode electrode design, the cathode electrode is prepared using two distinct and different cathode active materials and two cathode current collectors. The first cathode active material is sandwiched between the two current collectors and this assembly is, in turn, sandwiched between two layers of the second cathode active material.
A cross-sectional view of a sandwich cathode electrode assembly is presented in FIG. 2. This figure shows an electrode assembly 30 including an anode electrode 32 and a cathode electrode 34 segregated from each other by separator sheets 36. The anode electrode comprises an anode active material 38, such as lithium, contacted to at least one side of an anode current collector 40, such as of nickel. In that respect, the anode electrode 32 of the electrode assembly 30 is the same as the anode electrode described with respect to FIG. 1.
The electrode assembly 30 further includes the sandwich cathode electrode 34 having spaced apart cathode current collectors 42 and 44 with a first cathode active material 46 sandwiched between them. The cathode active material 46 is of a relatively high energy density but of a relatively low rate capability. A second cathode active material 48, different than that of the first cathode active material 46, is contacted to the opposite sides of the current collectors 42, 44. The second cathode active material is of a relatively low energy density but of a relatively high rate capability. This electrode assembly is the fundamental structure for an electrochemical cell having a sandwich cathode electrode. As with the electrode assembly shown in FIG. 1, the electrode assembly 30 is typically built in a case-negative design.
Since the sandwich cathode electrode design is completely different from conventional prismatic and jellyroll cathode electrode designs, the most efficient electrode assembly for conventional cells is not the most efficient assembly for cells with sandwich cathode electrodes. For this reason, the present invention discloses a new efficient cell stack design utilizing sandwich cathode electrodes in combination with half double screen sandwich cathode electrodes as the cell stack components. This new electrode assembly based on the sandwich cathode electrode design is termed a double screen sandwich cathode electrode design.
The present invention improves the performance of lithium electrochemical cells by providing a new electrode assembly based on a sandwich cathode design. The present invention is termed a double screen sandwich cathode electrode design. Cells powering implantable medical devices, such as a cardiac defibrillator, and utilizing a double screen sandwich cathode electrode have improved volumetric efficiency. In particular, the present invention uses sandwich cathode electrodes which are, in turn, sandwiched between two half double screen sandwich cathode electrodes, either in a prismatic plate or serpentine-like electrode assembly. In a jellyroll electrode assembly, the cell is provided in a case-positive design and the outside round of the electrode assembly is a half double screen sandwich cathode electrode.